A little more information would be helpful - type of food? price range? In Clinton itself or general vicinity?
In Clinton itself, I like Dora restaurant. According to their website, they are open 7 days for lunch and dinner.
There are many other choices, but before I list them, perhaps give a little more information about what you are looking for.
My first choice within your parameters would still be Dora's, but other options in Clinton include:
1. The Fine Diner - - Despite the name it is NOT a Diner but a small casual restaurant with very nice food at reasonable prices. Lunch on Sundays 11 am - 3 pm. Lunch Menu at: http://www.thefinedinernj.com/lunch.pdf
2. The Clinton House - Historic Inn. Lunch on Sundays beginning at 12 noon. I've lived in Clinton area (including in downtown Clinton itself) for 15 years yet have never made it to the Clinton House. Not sure why - - I hear good things about it from a colleague who goes there about once a month. http://www.theclintonhouse.com/index....
3. If scenery is important, Nino's is in a beautiful location on the river, next to the bridge, with a great view of the Red Mill. However, the food is not great. Nino's was originally pretty good Italian restaurant when it was on the corner of Leigh & Main Street. It then moved to the current location (where the Old Riverhouse restaurant used to be) and the food (IMHO) has not been particularly good. Not god-awful. But not good.
The Fine Diner
1 W Main St, Clinton, NJ 08809
The Clinton House
2 W Main St, Clinton, NJ 08809